1. Field of the Invention
The present disclosure relates to an optical sheet, and more particularly, to an optical sheet, which includes a prism pattern having condensing axes crossing each other in diagonal directions with respect to an arrangement direction of light sources, thereby achieving an enhanced contrast ratio without a reduction in viewing angle and an increase in costs, and a backlight unit using the same.
2. Discussion of the Related Art
Cathode Ray Tubes (CRTs) are one of generally used display devices and have been mainly used in monitors of televisions, measuring instruments, information terminals, and the like. However, CRTs have problems associated with being both heavy and bulky and thus, could not positively cope with necessities for smaller size and lighter weight of electronics.
Accordingly, CRTs have limits in size, weight, and other characteristics and cannot satisfy demand for ever lighter and smaller electronics. Possible substitutes for CRTs include Liquid Crystal Displays (LCD) using electric-field optical effects, Plasma Display Panels (PDPs) using gas discharge, Electro-Luminescence Displays (ELDs) using electric-field light-emission effects, and the like. Of these substitutes, research on LCDs is actively progressing.
LCDs have several advantages, such as e.g., small size, light weight, and low consumption of electric power, required to substitute for CRTs. Recently, LCDs have been actively developed to sufficiently perform the role of flat panel display devices, and are used in monitors of laptop computers, desktop computers, and other large-scale information display devices. Demand for LCDs is increasing continuously.
Most of such LCDs are light receiving devices that display an image by adjusting the quantity of light introduced from the outside and therefore, have an essential need for a light source, i.e. a backlight unit to irradiate light to an LCD panel.
Generally, a backlight unit, used as a light source of an LCD, is classified into an edge type backlight unit and a direct type backlight unit, based on the arrangement of a cylindrical fluorescent lamp.
Development of a direct type backlight unit began in earnest as LCD sizes increased above 20 inches. In a direct type backlight unit, a plurality of lamps is arranged in a line beneath a diffuser plate to directly irradiate light to a front surface of an LCD panel.
In an edge type backlight unit, a lamp unit is installed at a side surface of a light guide plate that serves to guide light. The lamp unit includes a lamp emitting light, lamp holders coupled to opposite ends of the lamp to protect the lamp, and a lamp reflector surrounding an outer peripheral surface of the lamp, one side of the lamp reflector being fitted to the side surface of the light guide plate so as to reflect the light emitted from the lamp toward the light guide plate.
Both types of the above described backlight units may further include a prism sheet having a condensing function. The prism sheet may be located above a light source or a light guide plate, and serves to condense light emitted from the light source therebelow and transmit the condensed light to a light exit surface.
Hereinafter, a conventional prism sheet will be described with reference to the accompanying drawings.
FIGS. 1A and 1B are views illustrating different prism sheets respectively having a horizontal condensing axis and a vertical condensing axis.
FIG. 1A illustrates a prism sheet 1 in which prism peaks are repeated in a horizontal direction, each prism peak extending in a direction penetrating the drawing. In this case, light condensing is performed in such a manner that light emitted from a light source beneath the prism sheet is collected in a horizontal direction.
FIG. 1B illustrates a prism sheet 2 in which prism peaks are repeated in a vertical direction, each prism peak extending in a horizontal direction. In FIG. 1B, the right side drawing is a cross sectional view taken along a direction passing an apex of the horizontally extending prism peak, and a dotted line represents a lowermost point of the prism peak. In this case, light condensing is performed in such a manner that light is collected in a vertical direction.
In the case of the above described prism sheets 1 and 2 in which the prism peaks extend in a given direction as illustrated in FIGS. 1A and 1B, although light condensing is performed in a direction along which the prism peaks are repeated, each prism peak exhibits deterioration in condensing effects in a longitudinal direction thereof. Due to such condensing characteristics in a specific direction, a viewing angle in a vertical direction and a viewing angle in a horizontal direction may have different characteristics.
The above described conventional prism sheets for backlight units have the following problems.
In the conventional prism sheets for backlight units, the prism peaks are arranged parallel to one another to have directivity in a specific direction, thus achieving high condensing effects in a given direction, but having poor condensing efficiency in another direction. This causes a difference in characteristics between a viewing angle in a horizontal direction and a viewing angle in a vertical direction.
To solve the above described problem, there has been attempted a method in which optical sheets are arranged in such a manner that a prism sheet having vertical directivity and a prism sheet having horizontal directivity cross each other, thus removing a difference in characteristic between a viewing angle in a horizontal direction and a viewing angle in a vertical direction. However, this method problematically increases a thickness and manufacturing costs of the resulting backlight unit.
Moreover, simply arranging the prism sheets to cross each other fails to improve the vertical and horizontal viewing angles up to an intended level, and providing a plurality of optical sheets causes a higher cost burden than an acquired efficiency. In addition, defects, such as stripes, etc., may occur at intersections of the prism peaks.